Ultra-violet (UV) lies beyond the blue end of the visible spectrum. UV photons have
higher energies than optical photons and are emitted by hot objects. Young stars in
particular shine brightly at UV wavelengths because they are very hot, their surface
temperatures can reach over 10,000 degrees Celcius.

The Earth's atmosphere blocks out most of the UV light from space - a very
good thing as the small amount of UV from the Sun which does pass through the
atmosphere is what causes skin to burn on a sunny day. If the atmosphere let
all UV light through then we would all be burned to a crisp! Because of this, UV
telescopes need to be above the atmosphere to carry out observations, so most UV
observatories are satellites.